Where a grinding wheel is rotated to grind or polish a workpiece, the grinding wheel becomes worn as a result of its functioning, or grinding filings enter between grains of the grindstone, as a consequence of which blinding or filling-up occurs, resulting in a lowering of grinding efficiency. Therefore, the grinding operation is discontinued every fixed time, and the surface is shaved by a grindstone tool called a dresser to project the grindstone grains on the surface.
This grindstone operation is generally called dressing. For the dressing of a grinding wheel for precise grinding, a diamond dresser is used. In such a case, it is necessary to pay sufficient attention to setting of a dressing with respect to the grinding wheel so that the surface of the grinding wheel is not excessively shaved, which requires skill.
Dressing by way of discharge is also well known. In either case, the diameter of the grinding wheel is decreased due to the dressing. It is therefore necessary to change the offset amount with respect to a work.
The change of the offset amount of the grinding wheel after dressing is carried out by artificially placing the grinding wheel into contact with the surface to be worked. Confirmation of the contact between the grinding wheel and the surface to be worked is made by listening to an insignificant contact noise at that time or looking at sparks generated by grinding. In view of the foregoing, grinding work accomplished by the grinding wheel requires time as compared with cutting work, drilling work, etc. The decrease in diameter of the grinding wheel due to the wear of the surface is insignificant. The diameter after dressing may be measured by mechanical or electric means. However, it has been difficult to automatically set a new grinding-wheel position on the basis of the measured value.